1. Field of the Invention
The present invention relates to a power output apparatus, and more specifically to a power output apparatus mounted on a vehicle for outputting power to a drive shaft.
2. Description of the Related Art
Proposed power output apparatuses mounted on a vehicle include a motor for outputting power to a drive shaft of the vehicle, an engine, a generator for converting the mechanical power output from the engine to electrical power, a battery being charged with the electric power generated by the generator and being discharged to supply electric power required for driving the motor, and a control unit for controlling operation of the engine, the motor, and the generator (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 6-245320). The conventional power output apparatus detects the electric power input from the generator and the battery to the motor, calculates the mean electric power by dividing the input electric power by a predetermined time period, and corrects a target electric power to be generated based on the mean electric power. This procedure controls the engine and the generator in order to enable the state of the battery (state-of-cell, SOC) to approach a predetermined level.
It is that this structure realizes efficient use of the battery as well as reduction of its size and that less fluctuation of the charging amount extends the life of the battery.
The conventional power output apparatus controls the state SOC of the battery to the predetermined level, which prevents further reduction in size of the battery. In case that the motor regenerates large electrical energy, for example, when the vehicle goes down a long slope, the battery should be charged with a greater portion of the regenerated electric power, in order to attain the higher energy efficiency. In case that the motor consumes large electric power, for example, when the vehicle goes up a long slope, on the other hand, the battery should be discharged to ensure a sufficient supply of electric power required for driving the vehicle. In order to satisfy these requirements, the battery needs a relatively large capacity, which results in increasing the size of the battery.
The charge-discharge efficiency of the battery varies with the state SOC of the battery. When the state SOC of the battery around the predetermined level that simultaneously satisfies the two requirements has a lower charge-discharge efficiency, the energy efficiency of the whole power output apparatus can not be raised sufficiently.